Steel sheet intended for the manufacture of terrestrial motor vehicle parts are generally coated with a zinc-based metallic coating, for corrosion protection, deposited either by hot-dip coating in a zinc-based liquid bath or by electrodeposition in an electrolytic bath containing zinc ions.
Galvanized sheet intended for the manufacture of skin parts are then formed and assembled, so as to form a body in white, which is then coated with at least one paint coat, thereby providing greater corrosion protection and a good surface appearance.
For this purpose, conventionally, motor vehicle manufacturers firstly apply a cataphoretic coat to the body in white, followed by a primer paint coat, a base paint coat and, optionally, a lacquer coat. To obtain a satisfactory painted surface appearance, a total paint thickness of between 90 and 120 μm is generally applied, made up, for example, of a 20 to 30 μm thick cataphoretic coat, a 40 to 50 μm thick primer paint coat and a 30 to 40 μm base paint coat.
To limit the thickness of paint systems to less than 90 μm, certain motor vehicle manufacturers have proposed either to omit the cataphoresis step or else to limit the number of paint coats so as to increase productivity. However, at the present time, this thickness reduction of the paint system is always to the detriment of the final painted surface appearance of the part and is not implemented in industrial production.
This is because zinc-based coatings serving as base substrates have wavy surfaces, which can presently be compensated for only by thick coats of paint, otherwise the body parts would have an unacceptable “orange peel” appearance.
The waviness W of the surface is a gentle, pseudoperiodic, geometric irregularity of quite a long wavelength (0.8 to 10 mm), distinguished from the roughness R, which corresponds to geometric irregularities of short wavelengths (<0.8 mm).